This invention relates generally to an electron lens. More particularly, it relates to a magnetic field type electron lens. This electron lens is used for converging or expanding charged particle beams in an electron microscope, an electron beam apparatus (EB), an ion beam apparatus, and so forth.
The magnetic field type electron lens in general converge or expand an electron beam by causing currents to flow through exciting coils disposed inside a magnetic path to generate magnetic fields.
When an electron lens having a high intensity is necessary, the currents to be caused to flow through the exciting coils are increased, or the number of turns of the exciting coils is increased. To increase the exciting currents, however, a power supply having the capacity of supplying greater currents is necessary. When the number of turns of the exciting currents is increased, a high voltage must be generated because the resistance of the exciting coils increases.
To solve these problems, JP-A-3-20950, for example, discloses a technology which disposes a plurality of exciting coils inside the same magnetic path, connects these coils in parallel with one another, and causes current to flow through them so as to resistrict the rise of a power supply voltage, and the increase of the current capacity while increasing the number of turns of the exciting coils and lowering their resistance.
In the prior art technology described above, however, when the currents passed through the exciting coils are changed so as to change the intensity of the electron lens, power that is consumed by the coils is proportional to the square of the exciting current unless the resistance of the exciting coils changes. In other words, when the exciting current is increased to increase the intensity of the electron lens, the magnetic path or in other words, a casing of the coil, undergoes thermal deformation due to the heat generated in accordance with this power. This deformation adversely affects the intensity and distribution with respect to the magnetic field of the electron lens and invites problems such as an irregular drift of an electron beam.
The prior art related with the present invention is disclosed in U.S. Pat. No. 4,306,149.